The present invention relates to an air-fuel ratio control system for an automotive engine provided with a carbon canister, and more particularly to a system having an electronic fuel injection system controlled by a learning system.
In one type of electronic fuel injection control disclosed in Japanese Patent Application Laid-Open 60--93150, the quantity of fuel to be injected into the engine is determined in accordance with engine operating variables such as mass air flow, intake-air pressure, engine load and engine speed. The quantity of fuel is determined by a fuel injector energization time (injection pulse width).
Generally, a desired injection quantity is obtained by correcting a basic quantity of injection with various correction or compensation coefficients of engine operating variables. The basic injection pulse width T.sub.p is expressed, for example, as follows. EQU T.sub.P =K.times.Q/N
where Q is the mass air flow, N is the engine speed and K is a constant.
Desired injection pulse width T.sub.i is obtained by correcting the basic injection pulse T.sub.P with coefficients for engine operating variables. The following is an example of an equation for computing the actual injection pulse width. EQU T.sub.i =T.sub.p .times.COEF.times..alpha..times.K.sub.L +T.sub.s
where COEF is a miscellaneous coefficient comprising various correction or compensation coefficients obtained from memories dependent on coolant temperature and throttle position, .alpha. is a air-fuel ratio correcting coefficient which is obtained from the output of an O.sub.2 -sensor provided in an exhaust passage, and K.sub.L is a correcting coefficient by learning (hereinafter called learning coefficient) for compensating the change of characteristics of devices with time in the fuel control system such as, injectors, air flow meter and pressure regulator, due to deterioration thereof, T.sub.s is a constant for compensating voltage. The coefficient K.sub.L are stored in a lookup table provided in a non-volatile RAM in a microcomputer, and a necessary coefficient is derived from the table in accordance with sensed informations. The learning is executed in steady states of the engine operation. In order to detect the steady state, an operation matrix comprising a plurality of divisions is provided. The column and row of the matrix represent engine operating conditions such as engine speed N and basic injection pulse width T.sub.P and each division is designated magnitudes of engine speed and pulse width. When the magnitudes of the engine operating conditions continue for a period of time within one of divisions, it is determined that the engine is in a steady state. In such a steady =state, the learning operation is executed. In the learning, the learning coefficient K.sub.L corresponding to the engine operating conditions is rewritten with a new coefficient which is obtained by incrementing or decrementing the coefficient with a value relative to the feedback signal from the O.sub.2 -sensor.
The learning starts when the output of the O.sub.2 -sensor changes cyclically, over a reference value for dividing a rich side and lean side, a predetermined number of times (three times) while the magnitudes of the engine operating conditions stay in one of the divisions in the matrix.
On the other hand, the engine is provided with a carbon canister for absorbing the fuel vapor in a fuel tank during the time when the engine is not running, and for purging the fuel vapor from the canister to an intake manifold in predetermined conditions of the engine operation. When the fuel in the canister is purged, the fuel vapor is added to the air-fuel mixture induced in cylinders of the engine, rendering the mixture rich.
When the vehicle is driven where the atmospheric temperature is high, or at high altitude, or when gasoline having a high vapor-pressure is used, a large amount of fuel vapor is generated. Consequently, when the canister is purged, the air-fuel ratio becomes excessively rich. Accordingly, the air-fuel ratio control system operates to dilute the rich mixture in accordance with the feedback signal of the O.sub.2 -sensor. Namely, the air-fuel ratio correcting coefficient .alpha. is set to a minimum value. At the same time the learning coefficient K.sub.L is updated so as to converge the output of the O.sub.2 -sensor on 1.0. Thus, the air-fuel ratio is maintained at a stoichiometric air-fuel ratio by extremely small amount of fuel. Under such a condition, when the throttle valve is closed at idling, the purge is cut off. Accordingly, the air-fuel mixture induced in cylinders immediately becomes lean. The learning control system operates to enrich the mixture by increasing the learning coefficient in response to the output of the O.sub.2 -sensor. However, since the learning coefficients are rewritten little by little, the air-fuel mixture stays lean for some time, which will cause malfunction of the engine.
Japanese Patent Application Laid-Open 61-112755 discloses a system where two learning coefficient tables are provided to solve such a problem. The learning coefficients are derived from the first table when the fuel vapor is purged from the canister and from the second table when the purge is cut off. However, in order to provide two tables, the capacity of a back-up RAM must be increased. In addition, the operation becomes complicated as that a microcomputer having a large capacity must be provided, thereby causing increase of the cost for manufacturing the control system.